This invention relates to the treatment of steel plant waste materials for recycling in the steelmaking operation.
In a typical steelmaking operation, an initial charge containing natural iron ore, sintered ore, or pelletized ore, or a mixture thereof, is smelted with fuel and flux in a reduction furnace, e.g., a blast furnace, and the smelted crude iron product is then subjected to one or more steelmaking and/or refining operations in a separate furnace or furnaces. In conventional steelmaking practice, the molten refining slag formed from impurities in the crude iron and added flux materials is separated from the refined steel, cooled, and processed for recycling to the smelting operation to recover metallic values contained therein. The resulting refined steel is normally subjected to steel rolling or forging operations.
A conventional rolling mill utilizes water emulsions of oil for lubrication and cooling purposes. Iron oxide scale is continually formed on the surface of the hot steel. This scale is repeatedly stripped away by the stresses of rolling. The scale falls, together with the emulsified oil, into collection pits beneath the rolling mill, whence the oil emulsion is recycled via sumps and filters. The scale accumulates and is periodically removed. This oil coated oxide scale is generally referred to in the art as "mill scale" or "roll scale".
The "mill scale" is a rich source of recyclable iron values, normally assaying from 65 to 75 percent iron, mainly as ferrosoferric oxide, Fe.sub.3 O.sub.4.
To recover the iron values, the scale must be re-smelted in a blast furnace or other reduction furnace. Mill scale, however, is usually too finely divided to enter the blast furnace without prior agglomeration. The conventional method for agglomerating mill scale to blast furnace charge size is to subject the scale to agglomeration by sintering. Thus, a bed of mill scale and other agglomerable iron oxide fines, together with fluxes, e.g. limestone and fuel, e.g. coke breeze, is placed on a moving grate which is subjected to suction draft from below and ignited by an ignition flame from the top. The sintering process flame consists of an intense but narrow flame front, sustained by the carbon and other combustibles in the mix, which passes downward from the ignited top progressively to the bottom of the bed as the grate passes slowly over successive wind boxes which sustain the suction draft. As the high-temperature process flame front passes downward through the bed, the moisture in the mix is flashed off, the carbonate fluxes are calcined, the carbon is consumed, and iron oxides and calcined fluxes fuse together in a cake which is subsequently broken up, cooled, screened to eliminate (and recycle) fines, and sent as a significant charge to the blast furnace for re-smelting.
The suction draft system draws considerable fines and dust out of the bed during the sintering operation such that the sinter grate exhaust is initially dirty. The said exhaust must be continually cleaned to avoid dust emissions, to minimize wear on the blades of the draft fans, and to recapture the dust for return to the sintering process. To clean the raw exhaust gas, multicyclones, venturi scrubbers, electostatic precipitators, and/or fabric filters, used singly or in combination, are interposed between the wind boxes and the drafting fan blades.
When oil-coated mill scale is introduced into this feed mixture of a sintering plant, however, the oil gives rise to substantial problems. The oil tends to vaporize well below its ignition temperature. It is thus driven out as a vapor with the moisture ahead of the advancing sinter process flame front in an unburned condition. The oil vapor tends to foul electrostatic precipitators and fabric filters, and may accumulate in part as a fire and explosion hazard. In addition, a substantial portion thereof often passes through the dust collection system and escapes through the exhaust stack as a condensed vapor commonly called "blue smoke". Although it is possible to engineer a venturi scrubber powerful enough to remove the oil mist, such systems are costly to install and operate. At the present time, it is considered more efficient to de-oil the mill scale before it is introduced into a sinter plant.
Several methods have previously been suggested for the removal of oil from mill scale. These include detergent washing, solvent extraction, or incineration of the oil in a fired or self-firing system. As an example of the latter, oil-coated scale has been passed through an inclined rotary kiln which is heated to drive off the oil as vapor through a special stack equipped with afterburners. Such approaches, however, suffer from the cost of initial capital installations which must be manned and maintained. In addition, such systems often do not result in complete ignition of the oil, thereby giving rise to substantial pollution of the environment.
It is an object of the present invention to provide a process for removing oil from mill scale in an economic, efficient and environmentally sound manner, and for recovering the metallic values contained in said de-oiled scale.